Spatial light modulators (SLMs) have long been used as input devices for coherent optical processors. They perform this function by spatially modulating a laser beam so that a two-dimensional image is encoded on the beam. One difficulty in using coherent optical processors in a wide variety of applications is the limitation of currently available spatial light modulators.
There are basically two approaches to SLM technology: magneto-optic devices and devices using liquid crytal material. Magneto-optic SLMs have high frame rates, excellent uniformity, and induce very little phase distortion in a laser beam. However, they are limited to binary states, i.e., transmitting or contransmitting, for each pixel. Liquid crystal devices on the other hand can represent a range of grey levels, but are much slower that magneto-optic devices, are less uniform, and introduce phase distortions which are proportional to the grey level at each pixel. As a result, magneto-optical devices are desirable from a practical viewpoint, but many applications require more than one bit of encoding per pixel.